Holding structure of piezoelectric vibrator and manufacturing method therefor

Abstract: Protrusions (13) made of an electrically conductive material such as a metal or ceramic are formed around a node point or a node line of vibrations of a piezoelectric plate (11) on two principal planes thereof with a spraying process. The piezoelectric plate (11) is held at the protrusions (13) with leaf springs on the two principal planes. Thus, the protrusions (13) of a small surface area are formed without deteriorating the piezoelectric plate (11). The piezoelectric plate (11) can be held stably without deteriorating vibration characteristics. An alternating voltage is applied to electrodes (12) of the piezoelectric plate (11) through the electrically conductive protrusions (13). Because a spraying process is used, the protrusions (13) can be formed thickly in a small area precisely and it held surely and reliably with good mechanical strength of the protrusions (13).

Description: The present invention relates to a holding structure of a piezoelectric vibrator comprising a piezoelectric plate made of piezoelectric material, which piezoelectric plate has electrodes formed on two principal planes thereof, protrusions made from an electrically conductive material and formed on a node of vibrations of said piezoelectric plate on each of the principal planes, a holder means for holding the piezoelectric plate at said protrusions formed on the principal planes of said piezoelectric plate. Such a holding structure is already disclosed in Japanese utility model laid-open publication 23617/1993. The present invention particularly relates to a piezoelectric vibrator made from a piezoelectric material such as a piezoelectric ceramic, quartz or lithium niobate, which can be used as a component of an oscillator, a ceramic filter or the like or in particular to a holding structure of the piezoelectric vibrator and a manufacturing method thereof. A piezoelectric vibrator is used for mechanical filters, oscillators, actuators or the like and it is made from a piezoelectric material such as a piezoelectric ceramic, quartz or lithium niobate. A suitable vibration mode such as bending vibration or extensional vibration is selected according to a frequency range to be used. When an alternating electric voltage is applied to a piezoelectric vibrator, it is vibrated due to the piezoelectric effect with a low vibration loss. Such a piezoelectric vibrator has a large mechanical quality factor Q m and a large coupling factor. Therefore, a ceramic filter can have a wider range and a vibrator has a better frequency precision than an LC filter using a capacitor and a coil. In order to utilize such features of a piezoelectric vibrator, it is important to support or fix a piezoelectric vibrator correctly and surely at a node where the vibration amplitude becomes zero so as not to suppress vibrations generated in the piezoelectric vibrator in a holding structure thereof. A node in a piezoelectric vibrator is a point or a line without zero area. Therefore, it is desirable that the holding structure holds a piezoelectric vibrator at a point or at a line. However, it is impossible practically to hold a piezoelectric vibrator at a point or at a line. Further, it is also needed to hold it stably. Then, a piezoelectric vibrator is practically held at a point-like area or at a linear area by sacrificing the characteristics thereof somewhat. Heretofore, piezoelectric vibrators were supported or fixed around a node by springs. However, this technique has following problems: (1) It is difficult to set positions for supporting or fixing a piezoelectric vibrator correctly around a node. (2) If a piezoelectric vibrator is supported or fixed strongly or a strong force is applied to the piezoelectric vibrator for supporting or fixing it surely, the characteristics of the piezoelectric vibrator are deteriorated or the piezoelectric vibrator is destroyed mechanically. (3) It is needed to support or fix a piezoelectric vibrator on a certain area in order to hold it at a particular posture. However, because a node of the piezoelectric vibrator is a point or a line with no area, this means that vibrating portions in the piezovibrator are held or fixed. Therefore, vibrations are suppressed and characteristics of the piezoelectric vibrator are deteriorated. Fig. 1 illustrates a concept of holding a piezoelectric vibrator 1 at points. Driving electrodes 2 are formed on two opposing principal planes of a piezoelectric vibrator 1 of a square plate made of a piezoelectric material. When an alternating voltage is applied to the driving electrodes 2, extensional vibrations of the square plate are generated. Because the vibrations has a node point at a center of the square plate, the piezoelectric vibrator l is held by holders 3 to interpose it at two sides with an applied pressure. A top of the holder 3 has a flat region with a certain area, and the piezoelectric vibrator 1 is held stably by abutting it to the flat regions of the holder 3 at two principal planes. Further, the elastic property of the holder 3 buffers vibrations propagated from the outside. Fig. 2 illustrates a concept of holding of a piezoelectric vibrator 1 at a line. Driving electrodes 5 are formed on two opposing principal planes of a piezoelectric vibrator 4 of a rectangular plate made of a piezoelectric material. When an alternating voltage is applied to the driving electrodes 5, longitudinal vibrations of the rectangular plate are generated having main displacements along a longitudinal direction of the rectangular plate. Because the vibrations has a node line at a center line of the rectangular plate, the piezoelectric vibrator 4 is held at two principal planes with an applied pressure by interposing it between two holders 6. A top of the holder 6 has a flat region with a certain area, and the piezoelectric vibrator 4 is held stably by abutting the flat regions thereto at two sides. Further, the elastic property of the holder 6 buffers vibrations propagated from the outside. As described above, prior art holding structures hold a piezoelectric vibrator directly with holders at point-like areas or at line-like areas having a certain area. Therefore, if the contact areas for holding are decreased, holding becomes unstable. Further, when a shock is given for example by dropping it to the ground, the posture or position is liable to change and this causes vibration characteristics to change largely. On the other hand, if the holding area is increased, a part of vibrating region is pressed and both mechanical quality factors and coupling factors decrease to deteriorate vibration characteristics. Because a piezoelectric vibrator is held directly with holders, a contact area between the vibrator and the holder and a holding position of the holders vary for each piezoelectric vibrator. Then, it is also a problem that the characteristic of the piezoelectric vibrator scatters. To sum up, it is difficult to hold a piezoelectric vibrator stably and reliably without affecting vibrator characteristics. In order to solve the above-mentioned problems, it is proposed to form protrusions at positions for supporting or fixing a piezoelectric vibrator and to support it at the protrusions. This technique has an advantage that if protrusions can be formed correctly at a node, the position for supporting or fixing can be set surely around the node. Heretofore, such protrusions were formed by a hard-soldering process, a soldering process or a plating with metallic material and by a printing process with an organic electrically conductive paint. The protrusions were made of an electrically conducting material because electrical leads are connected at the protrusions. Japanese Utility Model laid open Publication 20422/1993 proposes a holding structure of piezoelectric vibrator wherein protrusions made of electrically conductive rectangular rubber are formed around a node point of vibrations of a piezoelectric vibrator and the piezoelectric vibrator is held at the protrusions. However, this holding structure has a problem that protrusions are deformed with a pressure applied for holding. Then, a contact area between the vibrator and the holders are not constant among piezoelectric vibrators, and the characteristics of the piezoelectric vibrator scatter broadly. Japanese Utility Model laid open Publication 23617/1993 proposes a holding structure of piezoelectric vibrator wherein metallic protrusions are formed around a node of vibrations of a piezoelectric vibrator and the piezoelectric vibrator is held at the protrusions. It is proposed to produce protrusions by soldering, welding, melting, adhesion or the like. However, when the protrusions are produced with a technique accompanying heating, the temperature of the piezoelectric vibrator is liable to be heated to deteriorate characteristics of the piezoelectric vibrator. Especially, the deterioration is large for a piezoelectric vibrator made of a piezoelectric ceramic having a low Curie temperature. Generally, the Curie temperature of a piezoelectric ceramic is 250 - 350 °C, and when a piezoelectric vibrator is heated for a long time at say 125 °C or above, the characteristics are deteriorated largely. For example, if a piezoelectric ceramic of 300 °C of Curie temperature is heated for an hour, the piezoelectric constant is found not to change when heating at 140 °C or lower, but to change when heating above it. It is also found that the piezoelectric constant decreases in a shorter time with increasing heating temperature. If the metallic protrusions are formed with adhesion without heating, it is a problem that an adhesive agent extends outside the contact areas. This also scatters and deteriorates the characteristics of the piezoelectric vibrator largely. An object of the invention is to provide a new holding structure of piezoelectric vibrator which can hold a piezoelectric vibrator reliably and stably without deteriorating vibration characteristics. It is also an object of the invention is to provide a method of producing the new holding structure of piezoelectric vibrator. In accordance with the invention, a holding structure of a piezoelectric vibrator as defined in the preamble of claim 1 is characterized in that the protrusions have an outer contact surface with a roughness of at least 2µm, and that the holder means contact the contact surfaces of the protrusions by elastic pressure contact only. A preferable method for manufacturing a holding structure of such a piezoelectric vibrator comprises the steps of (a) providing a piezoelectric plate made of piezoelectric material, (b) forming electrodes on two principal planes of said piezoelectric plate, and (c) forming protrusions by depositing particles of an electrically conductive material on said electrodes on the two principal planes of said piezoelectric plate. Preferable embodiments are defined in the dependent claims. If the protrusion are formed for example by a spraying process, the electrically conductive hard protrusions are formed without heating. A holder means holds the piezoelectric plate at the protrusion on each of principal planes of the piezoelectric plate. A contact area between the piezoelectric plate and the holder means is kept small, and the contact is kept strong. Even if a relative position between them is displaced somewhat, a holding position viewed from the piezoelectric plate is kept constant. An advantage of a holding structure of piezoelectric vibrator of the present invention is that the excellent characteristic of a piezoelectric vibrator is kept stable and has small scattering. An advantage of a manufacturing method of a piezoelectric vibrator of the present invention is that a holding structure of piezovibrator can be formed at a low temperature without affecting the vibration characteristics of the piezovibrator. These and other features of the present invention will become clear from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings, and in which: Fig. 1 is a schematic perspective view of a prior art piezoelectric vibrator held at points; Fig. 2 is a schematic perspective view of a prior art piezoelectric vibrator held at lines; Fig. 3 is a graph of a characteristic of vibrant frequency plotted against applied pressure; Fig. 4 is a graph of external impact force resistance plotted against roughness at surfaces of protrusions formed on a piezoelectric vibrator element; Fig. 5 is a perspective view of a piezoelectric vibrator element of a first embodiment of the invention; Fig. 6 is a sectional view of a holding structure for the piezoelectric vibrator shown in Fig. 5; Fig. 7 is a partial view of a protrusion and a holding member; Fig. 8 is a partial perspective view of a holding member; Figs. 9A, 9B and 9C are sectional views of steps for illustrating an example of a manufacturing method of the holding structure of piezoelectric vibrator; Figs. 10A, 10B and 10C are sectional views of other examples of forming sprayed films; Fig. 11A and 11B are sectional views of steps of a different method of forming a sprayed film; Figs. 12A and 12B are top and bottom views of a piezoelectric vibrator with protrusions of different widths; Figs. 13A and 13B are top and bottom views of a piezoelectric vibrator with three protrusions; Fig. 14 is a perspective view of a disk-like piezoelectric vibrator; Fig. 15 is a sectional view of a holding structure for the piezoelectric vibrator shown in Figs. 12A and 12B; Figs. 16A and 16B are a top view and a bottom view of an example of a piezoelectric vibrator of a rectangular plate of a third embodiment of the invention; Figs. 17A and 17B are a top view and a bottom view of a piezoelectric vibrator of a rectangular plate of another example of the third embodiment; Fig. 18 is a perspective view of a fifth embodiment of the invention; Fig. 19 is a sectional view of the holding structure shown in Fig. 18 in the width direction along the bridge portion; Fig. 20 is a sectional view of a holding structure in the longitudinal direction; Fig. 21 is a partial view of the holding structure around a protrusion; Figs. 22A, 22B and 22C are sectional, views of manufacturing steps of a holding structure of piezoelectric vibrator successively; Figs. 23A and 23B are sectional views of a multilayer resist mask; and Fig. 24 is a perspective view of a piezoelectric vibrator element. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, wherein like reference characters designate like or corresponding parts throughout the views, embodiments of the invention are explained. In the present invention, protrusions are formed on a piezoelectric plate around a node of vibrations by depositing particles of electrically conductive material such as a metal or a ceramic with a spraying process or the like. Then, the piezoelectric plate is held by holder means by contacting the top of the protrusions with the holder means. Following advantages can be realized by using protrusions formed by using a spraying process or the like: (1) When the protrusions are formed, the temperature of a piezoelectric plate does not become so high as to deteriorate the characteristics thereof. (2) The protrusions can be formed thick enough to stabilize a supporting or fixing posture and the holding means are prevented to contact with the piezoelectric plate even if the posture changes a little. Such thick protrusions can be formed thick in a short time with a spraying process because sprayed particles have larger sizes than particles deposited in vacuum deposition, sputtering deposition or the like. (3) The protrusions has a high mechanical strength and a high adhesion strength to prevent them to slide against the piezoelectric plate even if an external shock is applied to the piezoelectric plate on falling or the like. Because particles to be deposited as the protrusions have larger sizes in a spraying process than in a sputtering process or the like, they stick into the piezoelectric plate more strongly and more surely when the protrusions are formed. (4) The protrusions have rough surfaces because sprayed particles to be deposited as the protrusions have large sizes in a spraying process. Then, they can be stuck into a holding member like wedges so that they can support or fix the piezoelectric plate stably and surely even if they are not pressed so strongly to the holding member. (5) Protrusions can be formed only around nodes of vibrations precisely and with a fine pitch in order not to suppress vibrations. Next, effects of roughness of surfaces of the protrusions are explained in a holding structure of piezoelectric vibrator. When a piezoelectric plate is supported or fixed at the protrusions, a force applied to the protrusions have to be kept within a certain range in order to keep supporting or fixing conditions for the piezoelectric element constant. Fig. 3 shows a characteristic of vibrant frequency plotted against applied pressure when a piezoelectric plate is supported or fixed at protrusions formed with a spraying process on the piezoelectric plate. If the applied pressure is P 1 or lower, the supporting or fixing of the piezoelectric plate is unstable. Then, vibrant frequencies scatter largely and characteristic thereof are also unstable. On the other hand, if the applied pressure is P 2 or higher, losses due to the supporting or fixing become large and characteristic of the piezoelectric plate becomes unstable again, or the piezoelectric plate is broken mechanically. It is needed to keep the applied force at the protrusions within a certain range or between P 1 and P 2 so as to keep supporting or fixing conditions constant roughly. The values of P 1 and P 2 vary with a material, a size, a vibration mode or the like of the piezoelectric plate or with a material, a size, a shape or the like of a holding means. It is desirable that an applied pressure is set at a lower value within the range in order to enhance the reliability of the piezoelectric plate. The protrusions have an effect to set an applied pressure at a lower value within the range. Fig. 4 shows durability against external impact force plotted against roughness at surfaces of protrusions formed on a piezoelectric plate. The piezoelectric plate is pressed at a value within the above-mentioned range of applied pressure for realizing stable characteristics. The ordinate represents an upper limit of an external impact force which does not affect the characteristics of the piezoelectric plate. If the roughness at the surfaces is less than 2 µm, an external impact force changes supporting or fixing structure and affects the characteristics of the piezoelectric plate. On the contrary, if the roughness is 2 µm or larger, the rough surface sticks into a holding member like wedges and the characteristics of the piezoelectric plate are not deteriorated if the external impact force becomes large. The value of the permissible limit of the external impact force vary with a material, a size or the like of the protrusions or with a material, a shape or the like of the holding member. The characteristics of the piezoelectric plate become stable if the roughness is 2 µm or larger though the lower limit of the roughness depends a little on the applied pressure. Though the upper limit is not shown in Fig. 4, it is understood that the roughness does not exceed the width of a protrusion itself. A spraying process is used for forming protrusions on a piezoelectric plate made of a piezoelectric material. After electrodes are formed on two principal planes or the piezoelectric plate, electrically conductive protrusions are formed on the electrodes at a node of vibrations of the piezoelectric plate. In an example of the spraying process, particles of sizes between 5 and 300 µm of a starting material are passed through a plasma at a high temperature so that they are melted. Then, the melted particles are blown along a gas stream to a piezoelectric plate for them to be deposited to form protrusions. The size of the particles of a starting material is selected appropriately according to a material, a size and the like of the protrusions and the roughness at the surface, while the plasma temperature is determined according to a material of the protrusions, roughness at the surfaces, adhesion strength and the like. The conditions for forming protrusions are selected to satisfy following conditions: (1) Temperature increase of the piezoelectric plate caused by a spraying process is kept as low as possible. (2) A time needed for forming protrusions is short as much as possible. (3) A mechanical strength of the protrusions is large. (4) Roughness at surfaces of protrusions are controlled. (5) Precision of adhesion position of the protrusions is good, and a fine pitch if necessary can be realized. Though a spraying process is used in the embodiment of the invention, other methods can be used if large melted particles can be deposited to form protrusions on the piezoelectric plate around a node of vibrations. Metallic protrusions may be produced with plating in a vacuum deposition process or the like without heating in principle, but such protrusions may have a height as low as several micrometers practically. If a piezoelectric vibrator is held by using such low protrusions, the piezoelectric vibrator is liable to contact with holders to deteriorate vibration characteristics. If protrusions are produced by printing an organic material such as an electrically conductive paint, it is also difficult to produce high protrusions. Further, such protrusions have weak mechanical strength adhesion strength. Therefore, they are not suitable for holding a piezoelectric vibrator. A spraying process used in the embodiments does not have such disadvantages of a vacuum deposition process or an electrically conductive paint. Because electrically conductive protrusions is formed at or near a node of vibrations of a piezoelectric plate and the piezoelectric plate is held at the protrusions, a contact area between the piezoelectric plate and a member for holding the piezoelectric plate is determined as the top area of the protrusions. The contact area can be controlled so as to have a prescribed value. Therefore, the area needed for holding a piezoelectric vibrator can be increased as much as possible by considering effects on the characteristics of the piezoelectric vibrator. Even if the means for holding the piezoelectric plate is displaced somewhat relative to the piezoelectric plate, the holding position is constant as to the piezoelectric plate. Therefore, the deterioration of the characteristics of the piezoelectric plate can be decreased while the stability and reliability of the characteristics of the piezoelectric plate can be improved. Because the protrusions also act as electrical leads, a holding structure of piezoelectric vibrator can be made simpler. Embodiment 1 Fig. 5 shows a holding structure of a piezoelectric vibrator of a first embodiment of the invention. A piezoelectric plate 11 is a rectangular plate of 4.4 mm length, 1.4 mm width and 0.5 mm thickness and it is made of a piezoelectric material such as a piezoelectric ceramic, quartz or lithium niobate. Electrodes 12 for driving the piezoelectric plate are formed on two opposing principal planes of the piezoelectric plate 11. When an alternating electric voltage is applied to the electrodes 12, longitudinal vibrations having a main displacement along a longitudinal direction of the piezoelectric plate 11 are generated as displayed with double arrows "q" in Fig. 5. The resonant frequency of the vibrator is 455 kHz. A node line of the longitudinal vibrations is linear at a center of the rectangular plate 11. Protrusions 13 made from an electrically conductive material such as a metal or a ceramic are formed along the node line linearly with a spraying process. They have 1.4 mm length, 0.2 mm thickness and thickness between 50 and 70 µm. The roughness at the surfaces of the protrusions is 5 - 10 µm. In the spraying process, minute particles of a metal, a ceramic or the like are melted and pass through a plasma at a high temperature, and the melted minute particles are blown with a gas stream onto a base to form a film of the particles. Therefore, a film as thick as 5 - 10 µm can be formed on the substrate easily at a fast deposition rate. Because a film formed with a spraying process (called as sprayed film) is formed by continuous deposition of minute particles at high temperatures, adhesion strength between particles included in the film is strong. Further, though each particle to be deposited is melted at a high temperature, the heat capacity of the particle is small, while the particles are blown to the substrate along a gas stream. Therefore, it is a feature that temperature increase of the piezoelectric plate is very small if compared with a technique such as a soldering or welding. The width and the thickness of the protrusions 13 are determined suitably by considering stable holding of the piezoelectric plate 11 and an effect on the characteristics of the piezoelectric plate 11 by taking the vibration resistance and shock resistance into account. It is desirable that the width and the thickness of the protrusions 13 are large to some extent in order to hold the piezoelectric plate 11 stably, while the width thereof is desirable to be smaller as to an aspect of the effect on the characteristics of the piezoelectric plate. Then, the width and the thickness of the protrusions 13 are determined appropriately by considering the two requirements to some extent. It is to be noted that the protrusions are formed with a spraying process in the embodiment. Then, the protrusion can be stuck to the piezoelectric plate 11 at a sufficient adhesion strength, without heating the piezoelectric plate 11 as in a welding process. The temperature of the plate increases actually to a temperature sufficiently lower than about 130 °C which begins to cause characteristic deterioration. Therefore, the temperature increase on forming the protrusions 13 is sufficiently lower if compared with prior art soldering technique, and effects on the characteristics of the piezoelectric plate is hardly recognized. Further, the protrusions can be formed easily as thick as 100 µm or more. Because the film can be formed so thick, the piezoelectric plate is not likely to contact with a holding means even if it is inclined to some extent. Then, the characteristics of the piezoelectric plate can also be kept stable from this aspect. It is required that the protrusion 13 is stable mechanically when a force for holding is applied and that the protrusion 13 has a stable electrical characteristic because it plays a role of an electrical lead for applying a driving voltage. Then, a material of the protrusion 13 can be determined appropriately by taking into account corrosion resistance, electrical conductance, elastic constant (hardness) and the like. The protrusion 13 is made from a material such as a metallic material such as a nickel-chromium alloy, a stainless steel alloy, molybdenum, brass, silver, a copper alloy or an aluminum alloy, or an electrically conductive ceramic. Fig. 6 is a sectional view along a longitudinal direction of an embodiment of a holding structure for a piezoelectric plate 11 shown in Fig. 5. The piezoelectric plate 11 is held above and below at protrusions 13 formed at the center thereof with a spraying process, by holders 14 having elastic properties and it is contained in a case 15. The elastic properties of the holders 14 are useful in order to hold the piezoelectric plate 11 at a constant applied pressure and to buffer external shocks or the like to prevent the propagation of external forces from a case 15 to the piezoelectric plate 11. The holder 14 in the embodiment is a leaf spring made of a spring material such as phosphor bronze or stainless steel. A leaf spring is provided having a length longer suitably than the length of the inner space in the case 15, and as shown in Fig. 6, two ends thereof are set at two ends of the inner space so as for the leaf spring to be bent like an arch. The leaf spring has 1.4 mm width and 100 µm thickness. A force applied to the piezoelectric plate above and below can be set at an appropriate value of 150 - 200 gf by designing elastic constants and size such as thickness and length appropriately. The protrusions 13 formed with a spraying process has a sufficient adhesion strength to the piezoelectric plate 11. It also has a sufficient mechanical strength which hardly causes elastic deforming when it is held by the holding members 14. Thus, the piezoelectric plate 11 can be held at the protrusions 13 so that it has good and reliable characteristics with small scattering. If the leaf springs are made from an electrically conducting material such as a metallic thin plate, an electrical voltage can be applied to the piezoelectric plate 11 through the leaf springs. The piezoelectric vibrator is used as a component of an oscillator, a ceramic filter or the like. An alternating voltage is applied to the electrodes 12 of the piezoelectric plate 11 through electrically conducting holding members 14 or through a supply means such as electrical leads formed on the holding members 14 and then through the protrusions 13. Preferably, the protrusions 13 are made from a relatively hard metallic material such as a nickel-chromium alloy, a nickel-chromium-aluminum alloy or molybdenum or an electrically conductive ceramic material. The protrusions are formed to have minute roughness of a few to a few hundreds micrometers, preferably from a few tens to a few hundreds micrometers, at the surfaces thereof. Further, the holding members 14 are made from a material such as brass, a copper alloy, an aluminum alloy having a hardness smaller than the protrusions 13. In a contact state formed in such a condition of the protrusion 13 with a holding member 14, as shown in Fig. 7, the minute roughness at the surface of the protrusion 13 sticks into the surface of the holding member 14. This mechanical bonding realizes a stable holding of the piezoelectric plate 11 to reduce the effects of vibrations and shocks well. Further, an adhesion area of a protrusion 13 to the piezoelectric plate 11 can be reduced to an extent that the mechanical bonding between the piezoelectric plate 11 and the protrusion 13 is enhanced. This improves the characteristics of the piezoelectric plate 11. In the example described above, the leaf spring as a holding member of the piezoelectric plate is made from a material having a relatively low hardness such as brass, a copper alloy or an aluminum alloy. A similar advantage can be realized in a modified example shown in Fig. 8. In the example, the leaf spring is made from a material having a relatively high hardness such as a stainless steel or a spring steel, while an electrically conductive film 14a is formed on a contact portion of a holding member 14 by using a material having a hardness lower than the leaf spring such as brass, a copper alloy, an aluminum alloy, a lead alloy or a solder. Then, the roughness at the surface of the protrusion sticks into the electrically conductive film 14a. An alternating voltage is applied to electrodes of the piezoelectric plate through the electrically conductive protrusions without using leads connected to the electrodes. The electrical conduction between the protrusions 13 to an electrical supplier provided externally is performed through the holder 14 made of an electrically conduction material or through an electrically conducting path formed along the holder 14. In the example explained above, the protrusion is formed on an electrode for driving a piezoelectric plate. However, the electrode may be formed after the protrusion is formed on the piezoelectric plate. Next, an example of a method for forming the protrusions 13 shown in Fig. 5 is explained with reference to Figs. 9A - 9C. As shown in Fig. 9A, an electrode 12 is formed on a piezoelectric plate 11, and an organic resist mask 197 is formed on the electrode 12 with screening printing. The resist mask has an opening of width "b" and height "t". The resist mask 197 is made of an organic material because a sprayed film formed with a spraying process has a tendency hard to adhere to a relatively soft material such as an organic material. Secondly, the resist mask 197 can be separated and removed completely with an etchant or the like without giving damages to the electrode 12 and a protrusion 13. Next, as shown in Fig. 9B, the protrusion 13 is formed as a sprayed film in the opening formed in the resist mask 197 at a node from above with a spraying process. The sprayed film has a height "h" equal to or lower than the thickness "t" of the resist mask 197. The sprayed film hardly adheres to the resist mask 197, and only sprayed particles adhere sparsely as shown schematically in a section Q in Fig. 9B. Therefore, the sprayed film is deposited only in the opening. Finally, as shown in Fig. 9C, the resist mask 197 is removed with a resist etchant. The resist mask 197 is removed completely, and the sprayed film remains as the protrusion 13. The protrusion 13 has a shape with definite sides or has a shape similar to that of the opening. That is, the protrusion 13 can be formed to have almost same width at upper and lower portions. The protrusion 13 has a flat top surface though it has minute roughness. Therefore, the piezoelectric plate 11 can be held not inclined against the holding member for holding it. It is also an advantage that a resist mask can be formed at a low cost. Next, a modified example is explained wherein a protrusion 13 has a width "a" at an upper side smaller than "b" at a lower side. Such a protrusion is needed according to following reasons: First, the width or area of the protrusion at the bottom is preferably as narrow as possible in order to decrease effects on the vibrations of the piezoelectric plate. Second, the width or area of the protrusion is preferably as wide as possible in order to prevent the piezoelectric plate to be inclined to make contact with the external electrode or the like. Figs. 10A - 10C show a step of three examples of a method of forming such a protrusion, in correspondence to a step shown in Fig. 9B explained above. In the three examples, an electrode 12 is formed on a piezoelectric plate 11, and an organic resist mask is formed on the electrode 12 with screening printing. The resist mask has an opening having a width "a" at the top wider than "b" at the bottom. However, the shape of the opening is different among the three examples. In a first example shown in Fig. 10A, the slope of the opening formed in the resist mask 197c is constant. In a second example shown in Fig. 10B, a resist mask 197d has an opening having a slope decreasing gradually up to the top. In a third example, a resist mask is composed of two layers 197e and 197f, and the size of the opening is different in the two layers. After the opening is formed, a sprayed film is deposited with a spraying process in the opening to form a protrusion 13. Figs. 11A and 11B show steps of a modified example of a method of forming a protrusion 13 having a wider portion at the top. As shown in Fig. 11A, a sprayed film is deposited over the top of the opening formed in the resist mask 197. That is, the height "h" of the sprayed film or a protrusion 13 is higher than the thickness "t" of the resist mask 197, in contrast to Fig. 9B. Then, the protrusion 13 made of a sprayed film flows over the opening to have the height "h" larger than "t" as mentioned above and simultaneously to extend to a width "d" wider than the width "c" of the opening. The top of the protrusion 13 has a width "e" larger than "c". Because the sprayed film is hard to be adhered to the resist mask 197, the size "d" does not increase so extremely, but is suppressed to an appropriate extension. Thus, the protrusion having a larger width at the top and a smaller width at the bottom can be produced. Then, the resist mask 197 is removed with an etchant, as shown in Fig. 11B, to form the protrusion 13 having a wider width at the top on the piezoelectric plate 11. The resist ink used in the examples is a resist X87 of Taiyo Inki Seizo KK. The resist is selected according to the width of the opening, the resist thickness, the resistance against heat and the like by considering that a sprayed film is hard to be adhered, that the resist film can be removed completely without causing damages to the electrodes 2, the protrusions 3 and the like and that the width and the thickness of the opening can be controlled to have desired values. The resist mask may be made of a dry film or a photoresist. In this case, a pattern of an opening is formed with a photolithography process or the like. The sizes such as a width of the two protrusions 13a and 13b formed on opposing principal planes of the piezoelectric plate 11 may be different from each other as shown in Figs. 12A and 12B, wherein reference numeral 36 denotes a position of a node line. In this example, the two protrusions 13a and 13b have widths different from each other. Then, the piezoelectric plate 11 can be held stably with protrusions of a smaller area than a holding structure using protrusions with the same width. The protrusions 13a and 13b extend along a whole width of the piezoelectric plate 11. However, even if they are formed along a part of the whole width thereof, similar effects can be realized. The number of protrusions 13 is not limited to two. Figs. 13A and 13B show an example wherein three protrusions 13c, 13d and 13e are formed on two principal planes of the piezoelectric plate 11. One protrusion 13c is formed on a principal plane along a node line as shown in Fig. 13A, while two protrusions 13d and 13e are formed on the other principal plane near the node line. In this example, the piezoelectric plate 11 can be held stably with protrusions of a smaller area than a holding structure using two protrusions to have less effects on the characteristics of the piezoelectric plate. Embodiment 2 Fig. 14 shows a disk-like piezoelectric vibrator of a second embodiment of the invention. Electrodes 21 are formed on two opposing principal planes of a disk-like piezoelectric plate 20, and when an alternating voltage is applied to the electrodes 21, extensional vibration modes having a main displacement along radial directions can be generated in the disk-like piezoelectric plate 20. A node of the vibrations is located at a center point of the disk-like piezoelectric plate, and electrically conductive protrusions 22 made of a metal or a ceramic are formed on the two electrodes 21 at the node point with a spraying process. A contact area of the protrusion 22 with the piezoelectric plate 20 is determined so as to hold the piezoelectric plate stably against vibrations and shocks and to have small effects on the characteristics of the piezoelectric plate 20. Generally, the area is 1/60 - 1/20 of the area of the piezoelectric plate 20. Fig. 15 shows a holding structure of the piezoelectric plate 20. The piezoelectric plate 20 is held above and below at protrusions 22 formed at the center thereof with a spraying process, by holding members 23 having elastic properties, and it is contained in a case 24. The plastic properties of the holding members 23 are useful in order to hold the piezoelectric vibrator 20 at a constant applied pressure and to buffer external shocks and the like to prevent the propagation of external forces from the case 24 to the piezoelectric plate 20. A piezoelectric vibrator of a square-like shape also may have a point node. In such a case, point-like protrusions are formed similarly to the above-mentioned case. In the first and second embodiments explained above, because electrically conductive protrusions are formed around nodes of vibrations of the piezoelectric plate with a spraying process, mechanical quality factors Q m and coupling factors of the piezoelectric plate remain large and the protrusions are strong mechanically. Then, a contact area between the piezoelectric plate and the holding member is determined by a top area of the protrusion and it is kept almost constant. Therefore, an area needed for holding the piezoelectric plate can be increased by considering the effects on the characteristics of the piezoelectric plate. Further, because the protrusions are formed on the piezoelectric vibrator, even if the holding member is displaced relative to the piezoelectric vibrator, the holding position is kept at the same position for the piezoelectric vibrator. Then, the characteristic of the piezoelectric vibrator remains constant and the piezoelectric vibrator can be held reliably and stably. Still further, because the protrusions also act as electrical leads for supplying an electrical voltage to the piezoelectric plate, an oscillator, a ceramic filter or the like can be constructed easily. Embodiment 3 Figs. 16A and 16B show a piezoelectric vibrator of a third embodiment of the invention including a plate 25 of a rectangular plate made of a piezoelectric material such as a piezoelectric ceramic, quartz or lithium niobate. Electrodes 26 for driving the piezoelectric plate are formed on two opposing principal planes of the piezoelectric plate 25. When an alternating electric voltage is applied to electrodes 26, longitudinal vibrations having principal replacement along a longitudinal direction of the piezoelectric plate 25 are generated. A node of vibrations of the longitudinal vibrations is linear at the center of the piezoelectric plate 25. As shown in Fig. 16A, point-like protrusions 27a made from a nickel alloy, a nickel-chromium alloy, molybdenum or the like are formed around the node designated with reference number 28 with a photolithography process and a spraying process. They are aligned in three rows, and each of them have a certain area. As shown in Fig. 16B, protrusions 27b are formed similarly around the node on the opposite principal plane of the piezoelectric vibrator 25, and they have areas larger than those of the protrusions 27a. Similarly to embodiments 1 and 2, the piezoelectric plate 25 is held above and below at the protrusions 27a, 27b by using holding members having elastic properties, and it is contained in a case. By controlling the size of the protrusions 27a, 27b provided on the two principal planes, the piezoelectric plate 25 can be held stably with smaller contact areas if compared with a piezoelectric plate having protrusions with same sizes. Thus, the piezoelectric plate can be held more stably and more reliably and have small effects on the characteristics of the piezoelectric plate. In the example shown in Figs. 16A and 16B, the protrusions have circular shape at the top thereof. However, a different shape such as a triangle, a square or a ring also act similarly for holding the piezoelectric plate with the holding members. It is also to be noted that protrusions formed on a piezoelectric plate having a contour other than a rectangle, such as a square shape or a circular shape or using extensional vibrations also operate similarly. Though the example uses a rectangular piezoelectric plate, it is understood that protrusions can also be formed on a piezoelectric plate of a shape other than a rectangle such as a square or a circle and similar advantages can be realized on the characteristics of the piezoelectric plate. Embodiment 4 Figs. 17A and 17B show a piezoelectric plate 25 of a rectangular plate of a fourth embodiment. This embodiment is same as the third embodiment except that the arrangement of protrusions 31a formed on electrodes 26 applied to a principal plane of the piezoelectric plate 25 are different from that of protrusions 31b formed on the other principal plane. That is, the positions of the protrusions 31b are set so as not to coincide with those of the counterpart protrusions 31a. Similarly to the third example, the protrusions have a different shape, other than a circle, such as a triangle, a square or a ring also act similarly for the holding of the piezoelectric vibrator with the holding members. It is also to be noted that protrusions formed on a piezoelectric vibrator having a contour other than a rectangle, such as a square shape or a circular shape, or using extensional vibrations also act similarly. Embodiment 5 Fig. 18 shows a perspective view of a holding structure of a piezoelectric vibrator of a fifth embodiment. A reference numeral 40 denotes a piezoelectric plate of a rectangular shape shown in Fig. 5. The piezoelectric element 40 has two linear protrusions 13 formed on the two principal planes thereof. A spacer 42 is made of a plate of a plastic material or the like and it has a large opening 43 having a size somewhat larger than the periphery of the piezoelectric plate 40. The piezoelectric element 40 can be placed easily by inserting it in the opening 48 of the spacer 42 without contact therewith. Then, the piezoelectric element 40 is interposed between two printed circuit boards 44 and 45 with a certain force as shown by arrows in Fig. 18. The boards 44, 45 also have openings 46 in correspondence to the opening 43 but they have a bridge portion 47 in correspondence to the linear protrusions 13. For electrical conduction through the protrusions 13, electrode patterns of copper foils 48, 49 are provided on the bridge portion 47 in planes to make contact with the protrusions 13. Fig. 19 shows a section of the holding structure shown in Fig. 18 in the width direction along the bridge portions 47. It is shown that the piezoelectric plate 11 is held at the protrusions 13 by the printed circuit boards 44 and 45. The thickness of the spacer 43 is determined so as to hold the piezoelectric element 40 with an appropriate force by taking into account flexural rigidity and flexure quantity of the printed circuit boards 44, 45. Electrodes for driving the piezoelectric plate 11 are connected to the copper foils 48, 49. Though the example uses a rectangular piezoelectric plate, it is understood that a similar holding structure can also be used for a piezoelectric plate of a shape other than a rectangle such as a square or a circle for extensional vibrations or a bending vibrations and causes similar advantages on the characteristics of the piezoelectric plate. As explained above, the shape of the holding structure of the fifth embodiment is designed to hold the piezoelectric plate stable at the protrusions. However, it is possible to hold the piezoelectric plate only by using the copper foils 48, 49 stably without forming openings in the printed circuit boards 44, 45. Embodiment 6 Fig. 20 shows a perspective view of a holding structure of a piezoelectric plate shown in Fig. 5. Suffixes "a" and "b" of reference numerals denote top side and bottom side, respectively. External electrodes 134a and 134b are formed on holding plates 135a and 135b, and the two protrusions 13 are contacted with and interposed by the two external electrodes 134a, 134b or by the two rectangular holding plates 135a, 135b. The plates 135a, 135b are fixed with a frame 137 provided at the periphery of the plates 135a, 135b. External terminals 136a and 136b are formed on two sides of the periphery of the frame 137 as shown in Fig. 20 and make contact electrically with the external electrodes 134a and 134b, respectively. Fig. 21 shows a partial view of the holding structure around the protrusion 13. The bottom of the protrusion 13 adheres to the electrode 12, while the top thereof sticks into the external electrode 134b as illustrated by the minute roughness in Fig. 21 schematically. Thus, the piezoelectric plate is held surely between the holding plates 135a, 135b, while the electrical conduction are kept surely between the external electrodes 136a, 136b and the driving electrodes 12. Next, a manufacturing method of the holding structure is explained with reference to Figs. 22A - 22C. As shown in Fig. 22A, a resist mask 138 for patterning a protrusion is formed first on a plane of a piezoelectric plate 11 on which an electrode 12 is formed for driving. The resist mask 138 has an opening patterned along a node of the piezoelectric plate 11. A film is formed in the opening with a spraying process by using a metal such as molybdenum, nickel or chromium or an alloy thereof or a composite material to form a protrusion 13. The protrusion 13 is formed on each of the principal planes of the piezoelectric plate 11. Next, as shown in Fig. 22B, a frame 137 is put on a holding plate 135b on which an external electrode 134b is formed, and on a prescribed position on the holding plate 135b as constructed above, a piezoelectric plate 11 is put via the protrusion 13 and the resist mask 138b. Then, another holding plate 135a with another external electrode 134a is placed thereon. Then, the holding plates 135a and 135b are fixed to a frame 137 with an adhesive agent or the like, while holding the piezoelectric plate in a prescribed position. In this state, because the resist masks 138 are formed on the piezoelectric plate 11, it does not contact with the external electrodes 134a, 134b directly even if they are inclined or displaced somewhat on fixing. Next, as shown in Fig. 22C, after constructing the piezoelectric plate 11 and the holding plates 135 as described above, the resist masks 138a, 138b are removed. Therefore, the piezoelectric plate 11 can be located and fixed at a normal position against the holding plates 135a, 135b, the frame 137 and the like. Because the piezoelectric plate 11 is held by the holding plates 135a, 135b in the same state before removing the resist masks 138, it does not make contact with the external electrodes 134, the holding plates 135, the frame 137 or the like. Therefore, the characteristics of the piezoelectric vibrator is improved and the stability thereof is also improved. Further, the construction and the adjustment can be performed efficiently to improve the productivity of a holding structure of piezoelectric vibrator. Next, the thickness T r of the resist masks 138 and the thickness T n of the protrusions 13 are explained with reference to Fig. 22A. In Fig. 22A, T r < T n . In concrete, a difference between T r and T n is set to be about 5 - 30 µm. Then, in a state shown in Fig. 22B, there exists gaps between the external electrodes 134a, 134b and the resist masks 138a, 138b, and an etchant liquid for removing the resist masks 138a, 138b is likely to penetrate through the gaps to make it easy to release them. Further, because T r < T n , the protrusions 13 make contact with the external electrodes 134a, 134b surely. The external electrodes 134 are made of copper having a hardness smaller than the protrusions 13, while the protrusions 133 has rough surfaces. Therefore, the roughness sticks into the external electrodes 134a, 134b, so that sure holding and sure electrical contact can be realized. The resist mask 138 are made of a resist which can be patterned with a photolithography process and can be dissolved or softened and removed with a releasing liquid used exclusively therefor. For example, a dry film photoresist of α, AF00, BF type is available from Tokyo Oka Kogyo KK. It is needed to provide a gap sufficient to pass the liquid or the removed resist mask without caught by the holding plates 135, the frame 137 or the like. Therefore, the liquid is preferably a type which can dissolve the resist mask 138. For example, a liquid of PMER P-AR 9000 is available from the Tokyo Oka Kogyo KK. The resist mask 138 may be formed with screening printing. In this case, a soluble etching resist ink X-87 or M-85 of Taiyo Inki Seizo KK may be used. A resist material is selected by considering following conditions: Corrosion, erosion or the like is not caused by a development agent or an etchant liquid on the piezoelectric plate 11, the driving electrodes 12, the external electrodes 134, the holding plates 135, the frame 137 or the like. A required thickness can be obtained easily. If a resist material is made from an organic material mainly and it satisfies the above-mentioned conditions, it can be used for forming the protrusions 13 beside the above-mentioned examples. A resist mask of a thickness of 30 µm or more in correspondence to the thickness of the protrusions 13 can be formed with a resist film. However, if an ink type resist is used, a resist mask can be made thick by providing a multilayer resist. In an example shown in Fig. 23A, a resist mask is formed of two resist layers 138c and 138d, and an opening in the top layer 138c is larger than that in the bottom layer 138d. In another example shown in Fig. 23B, a resist mask is formed of three resist layers 138e, 138f and 138g, but with openings of the same size in the three layers. Then, a protrusion 13c, 13d is formed in the openings. The top layers 138c, 138e are made of a soluble resist. Then, after the top layers 138c, 138e are dissolved, the gap is widened, and the liquid passes easily. Then, the lower resists 138d, 138f and 138g can be removed easily. A similar advantage can be realized if at least one layer in a multi-layer resist mask is made of a soluble resist. Embodiment 7 If an electrode for driving a piezoelectric plate is formed not along a whole width of a piezoelectric vibrator, but along a part of the whole width and polarization of the piezoelectric plate is performed by applying a voltage through the electrode, polarization is not generated in a portion on which the electrode is not formed. Then, imbalance of internal stress occurs inside the piezoelectric plate at an interface between the above-mentioned portion without electrodes and the other portion, and this may cause cracking in the piezoelectric plate. A seventh embodiment of the invention solves this problem on the polarization. As shown in Fig. 24, electrodes 12a' and 12b' are formed on a piezoelectric plate 11 partly in the width direction. Then, before forming protrusions 13a, 13b, electrically conductive resist mask 188a, 188b are formed on the whole principal planes except openings for protrusions. Next, the protrusions 13a, 13b are formed in the openings. The piezoelectric plate 11 is polarized with the use of the electrically conductive resist mask 188. Thus, the piezoelectric plate 11 can be polarized completely, including the portion on which the electrode 12a', 12b' are not formed. Because the polarization occurs uniformly inside the piezoelectric plate 11, no imbalance of internal stress is generated. That is, the above-mentioned problem is solved, and the characteristics of a piezoelectric plate is improved and the yield of a piezoelectric vibrator is improved. Next, the piezoelectric vibrator produced as described above is constructed as shown in Fig. 20 with the plates 135a, 135b and the like. Alternately, a piezoelectric vibrator obtained as described above is cut off by a portion on which the electrode 12a' and 12b' are not formed, and it is used to construct a structure as shown in Fig. 20. The protrusion 13 may be formed after the polarization of the piezoelectric plate 11 as described above. However, though the protrusion 13 is formed with a spraying process which causes a gradual temperature increase, the temperature of the piezoelectric plate 11 increase somewhat. Therefore, it is preferable to cause polarization after forming the protrusions 13. The resist masks 188a, 188b are made of a liquid resist material including a prescribed quantity of silver as electrically conductive material, but they may be made of a material which is electrically conducting, hard for a sprayed film to be adhered and can be subjected to patterning easily. The piezoelectric plate 11 itself is an insulator and has a high specific resistivity. Further, the resist mask 188 has a thickness narrower than the piezoelectric plate 11. Therefore, it is preferable that the specific resistivity of the resist masks 188 are smaller than that of the piezoelectric plate 11 for example by three digits. Then, an electrically conductive component is controlled to provide a specific resistivity which is equal to or below the above-mentioned condition. However, if the added quantity of the electrically conductive material is too large, characteristics such as patterning and elasticity are deteriorated. In such a case, it is added to an order about 1 mΩ·cm. The electrically conductive material may be a metal such as gold, nickel or molybdenum, an alloy thereof, a compound such as an carbide, a boride or an oxide, having an appropriate electrical conductance. Although the present invention has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications are apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims unless they depart therefrom.

Claims: A holding structure of a piezoelectric vibrator comprising: a piezoelectric plate (11; 20; 25; 40) made of piezoelectric material, which piezoelectric plate has electrodes (12; 21; 26) formed on two principal planes thereof; protrusions (13; 13a, 13b; 13c, 13d, 13e; 22; 27a, 27b; 31a, 31b) made from an electrically conductive material and formed on a node of vibrations of said piezoelectric plate on each of the principal planes; a holder means (14; 23; 44, 45, 47, 48; 134a, 135a, 134b, 135b) for holding the piezoelectric plate at said protrusions formed on the principal planes of said piezoelectric plate; characterized in that the protrusions (13; 13a, 13b; 13c, 13d, 13e; 22; 27a, 27b; 31a, 31b) have an outer contact surface with a roughness of at least 2µm, and that the holder means (14; 23; 44, 45, 47, 48; 134a, 135a, 134b, 135b) contact the contact surfaces of the protrusions by elastic pressure contact only. The holding structure according to claim 1, wherein said holder means (14) comprises a holding member made of an electrically conductive material having a hardness smaller than that of said protrusions (13) at least at a portion (14a) for making contact with one of said protrusions. The holding structure according to claim 1, wherein said piezoelectric plate (11) has a linear node of vibrations and said protrusions (13) are formed linearly along the linear node. The holding structure according to claim 3, wherein one (13c) of said protrusions (13) is formed on one of the principal planes, two (13d, 13e) of said protrusions (13) are formed on the other of the principal planes around the linear node, and the two (13d, 13e) of said protrusions have a smaller contact area each with said holding means (14) than the one (13c) of said protrusions. The holding structure according to claim 3, said protrusions comprising a plurality of point-like protrusions (27a, 27b) distributed along a linear node of vibrations of said piezoelectric plate (25). The holding structure according to claim 5, wherein positions of the point-like protrusions (31a) on one of the principal planes of said piezoelectric plate (26) do not coincide with those (31b) on the other of the principal planes. The holding structure according to claim 5, wherein sizes of said point-like protrusions (27a) on one of the principal plane of the piezoelectric plate (26) are larger than those on the other (27b) of the principal planes. The holding structure according to claim 1, wherein said piezoelectric plate (20) has a point node of vibrations and said protrusions (22) are formed at the point node. The holding structure according to claim 1, wherein said holding member comprises two plates (44, 45) for holding said piezoelectric plate (40) between them and a spacer (42) for locating said piezoelectric plate between the two plates (44, 45), and the two plates (44, 45) have a printed circuit (48, 49) for electrical conduction to said piezoelectric plate (40). The holding structure according to claim 1, wherein an area of said protrusion (13) is larger at a top thereof in contact with said holding means (14) than at a bottom thereof in contact with the electrode (12) formed on said piezoelectric plate (11). A method for manufacturing a holding structure of piezoelectric vibrator comprising the steps of: (a) providing a piezoelectric plate (11) made of piezoelectric material; (b) forming electrodes (12) on two principal planes of said piezoelectric plate; and (c) forming protrusions (13) by depositing particles of an electrically conductive material on said electrodes on the two principal planes of said piezoelectric plate (11). The method according to claim 11, further comprising the step of: (d) holding said piezoelectric plate (11) with holding means (14) at the protrusions (13) after the step (c). The method according to claim 11, wherein a spraying process is used in said step c) of depositing particles. The method according to claim 11, said step of forming protrusions comprising the steps of: (e) forming a resist film (197) on said piezoelectric plate (11); (f) providing an opening in the resist film (197) around a node of vibrations of said piezoelectric plate (11); (g) spraying an electrically conductive material onto said opening to form said protrusions (13); and (h) removing said resist film. The method according to claim 14, wherein the thickness of said resist film (197) is smaller than that of the height of said protrusions (13). The method according to claim 14, wherein the thickness of said resist film (197) is larger than that of the height of said protrusions (13). The method according to claim 14, wherein the resist film comprises a plurality of successive layers (197f, 197e) with different thickness from each other. The method according to claim 14, wherein the resist film is removed partially in said step (h). The method according to claim 14, wherein said electrodes (12a', 12b') are formed partially on the two principal planes of said piezoelectric plate (11), and said resist films (188a, 188b) are made from an electrically conductive material and are formed entirely on the two principal planes respectively in the step (e) and said piezoelectric plate (11) is polarized between the steps (e) and (h) by applying an electrical voltage between said resist films (188a, 188b) formed on the two principal planes. The method according to claim 11, wherein steps (b) and (c) are performed in reversed order, i. e. first forming protrusions (13) and then forming electrodes (12).

Inventors: FUKUSHIMA HIROSHI, KAWASAKI OSAMU, NAGASAWA MASAHIRO, SHINTAKU HIDENOBU, SUZUKI SHIGEO, TAJIKA HIROFUMI, FUKUSHIMA, HIROSHI, KAWASAKI, OSAMU, NAGASAWA, MASAHIRO, SHINTAKU, HIDENOBU, SUZUKI, SHIGEO, TAJIKA, HIROFUMI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1994
Main classification: B06B1
IPC codes: H03H9, B06B1